As is heretofore known in many applications, a driving belt is wound around a plurality of pulleys one of which is a driving pulley and the other ones of which are driven pulleys adapted to move various accessory devices. For example, a toothed belt can be inserted in the drive of a motor vehicle to transmit the motion from the driving shaft to the camshaft. Also, for example, a belt, in particular a V-belt, can be used in a motor vehicle to transmit the motion from the crankshaft to a plurality of pulleys which in turn are associated with various devices, among which are the engine cooling fan, the engine water pump, the alternator, or the compressor of the air conditioning system if present.
In general, the above type of belt, when used in many of the above-noted applications to connect various pulleys for driving the plural devices arranged in different positions, is oriented in an annular zig-zag configuration requiring the use of a suitable device known as a tensioner to exert on the loose branch of the belt a predetermined tensioning that guarantees the regular working of the drive.
Moreover, as known from U.S. Pat. No. 4,706,696, a tensioner can be formed mainly of a hydraulic cylinder inside which is disposed a piston immersed in a viscous liquid that, in turn, is associated with a stem projecting from the upper base of the cylinder. The piston part opposite to the stem is provided with a cylindrical spring applied between the lower base of the cylinder and the inner face of the piston. The piston is crossed by a hole, which in the rest position is closed by a ball thrust against the hole due to the action of an auxiliary opposite spring maintained in an appropriate seat on the lower surface of the piston. In this device the free end of the stem acts on an appropriate lever anchored at one end to a structure fixed with respect to the belt and having its free end associated with a roller which presses with a given force against the back of the belt. If belt slackenings occur due to instantaneous torque changes on the driving pulley or any other cause, the pressure exerted on the lower part of the piston by the spring suitable for tensioning the belt is no longer compensated by the pressure that the belt exerts in an opposite direction on the stem, so that the piston has a tendency to move upwardly causing the liquid to pass through the hole provided on the piston overcoming the stiffness of the counteracting spring which blocks the exit of the hole through the cited ball. In this tensioning device, the dimensions of the hole crossing the piston are conveniently chosen in order to reduce the resistance met by the liquid on passing through the hole itself and to facilitate the shifting of the stem upwardly so that the roller presses again on the back of the belt to restore correct tensioning of the latter.
In the event of increases in the belt tension caused, for instance, by a thermal expansion phenomena tending to move apart the supports of the two pulleys between which the belt branch thrust by the tensioner is arranged, the belt exerts on the stem a force in a direction opposite to that previously described in the loose condition. In this situation the tensioning device is adapted to damp the movements of the belt through a blow-by of liquid between the upper chamber and the lower chamber of the piston; the blow-by of liquid is obtained by means of a very narrow hydraulic passage between the piston and the cylinder wall. In practice, the thrust that the belt transmits to the stem enables the pressure of the liquid present in the lower chamber to maintain the ball against the hole crossing the piston, thus blocking this hole and forcing the liquid to pass from the lower chamber to the upper one through the very reduced space between the piston and the cylinder. In essence, a liquid rolling occurs to which a damping of the movement transmitted by the belt to the tensioner corresponds.
U.S. Pat. Nos. 4,411,638 and 4,466,803 disclose additional examples of tensioning devices. All these devices provide also a tensioning spring to shift the piston together with the stem for restoring correct tensioning of the belt and a recourse to a proper choice of two valves and of suitable hydraulic circuits connected to the valves. In substance, such devices operate such that the upward thrust of the piston is facilitated, while the thrust of the piston in opposite direction is slower. The upward thrust of the piston is facilitated because when one of the valves opens, this allows the passage of the viscous liquid from the upper chamber to the lower one, for instance, through a large-sectioned conduit, while to obtain damping of the belt movements, the second valve makes the liquid pass from the lower chamber to the upper one through a conduit having a section much narrower than that of the first conduit, or through a conduit provided with a narrowing so as to increase the resistance met by the liquid on passing between the two chambers.
Unfortunately, in all of these devices the spring intended to restore the belt tensioning is strictly connected to the viscous means and this retards the restoration of the desired tensioning. In fact, the valve and the relative circuit chosen with a suitable determination of the size to facilitate the step of maximum excursions of the stem determines a certain forcing of the liquid through the passage section between the two chambers and this in turn determines a slowing down in the action of the tensioning spring. This solution can result in a completely unacceptable drive and produces numerous risks for the integrity of the system to which the drive is connected. In fact, in the case of a toothed belt applied between the driving shaft and the camshaft, in the event of belt slackenings not immediately compensated by the action of the spring of the tensioner, a skipping phenomenon could occur in relation to one or more teeth of the belt from the teeth of the pulley by which the belt meshes, with the consequence of originating a displacement in the adjusting system of the valves set in action by the camshaft and in some circumstances irreparable damage to the motor vehicle engine.
Moreover, in all the applications in which recourse is made to a device for the transmission of motion between a belt and a pulley through the exchange of friction forces, a retardation in the intervention of the tensioning spring in the event of slackenings of the belt produces unavoidably a sliding phenomena between the belt and the surfaces of the pulleys, with the consequence of damaging the elastomeric material for the dissipation of energy due to the strong friction forces created.
Additionally, all of the heretofore known solutions are complex due to the use of valves and relative circuits that permit an asymmetric working of the tensioning device.